Does your kid like hot food for lunch? Do you? Are you frustrated that you have no way to heat your lunch when you’re on the go? A thermal food jar (or thermos) is an excellent solution! Thermoses or thermal jars — I’ll use the two terms interchangeably in this post — are designed to keep food that is already hot from cooling too quickly or to keep food that is cold from warming to an unsafe temperature before you can eat it.

Thermoses can be pricey and there is nothing more frustrating than spending your hard-earned cash on something that doesn’t work well. It pays to do a little research before choosing a thermal jar and I am here to help. I’ve spent the last yeartesting 10 different thermoses by packing them with real life lunches and sending them to school with my boys. I’ve also done several controlled experiments (well, as controlled as you can get in my kitchen!) to test the jars side by side for quality.

Thermos Basics

To get the best results from your thermal jar, you should plan to do a little preparation in the morning. Most jars promise to keep food hot for about 5 hours and cold for a few hours longer, so keep this information in mind along with the time you plan to eat your food when you are deciding when to fill your jar in the morning. I ran my tests with the idea that most people will let food sit in their thermoses about four hours, but your situation may vary, of course.

Hot Food: Before filling your thermal jar with food, pre-heat it by filling it with boiling water and letting it sit with the lid on for about 10 minutes. If you are in a rush, you can use very hot tap water or let it sit for less time but this will impact the jar’s effectiveness. Heat food so that it is piping hot and then add it to the jar and put the lid on immediately (empty the hot water out first). Remember: thermal jars do not heat food — they keep already hot food from cooling off too quickly.

Cold Food: Pre-cool your jar before filling it with cold food by putting it in the freezer with the lid off for about 10 minutes. Pull food straight from the refrigerator if possible and then add to the jar.

The Vacuum Effect: As hot food cools, it can create a strong vacuum in a thermal jar and make the lid very hard to open. Take care not to over-tighten the lid when closing your thermos and make sure that your child’s hands are strong enough to open it by themselves so they don’t end up going hungry because they can’t get to their food at lunch time. Try to close the lid just tight enough that it doesn’t leak.

The Nitty-Gritty Thermos Low-Down:

I tested 10 thermal jars, but only 7 made the final cut. Any of these thermoses would be a good choice — which one you choose is a matter of your needs regarding capacity, size, and appearance. All of these jars fit the following criteria:

Keep food at a safe temperature — I eliminated three of the thermal jars I tested because they didn’t keep food out of the USDA Danger Zone even under ideal circumstances. (They didn’t even come close.) Some food was outside that range by serving time but they were close so I included them anyway.

Stainless steel interior — I chose stainless over plastic containers because I prefer not to store my food in warm plastic. It’s also less likely to stain (duh!) or store odors and it has superior powers of heat retention. None of the plastic jars I tested even came close to keeping food warm enough.

Durable — I’ve used all of these thermoses for around a year and none of them has broken yet, despite being tossed in a drawer repeatedly and being roughly handled by two kids. Most of them have a few dents at this point but they are still perfectly functional.

The sleek look in fun, bright colors and the medium size are a good choice for teens and tweens.

The LunchBots 12 oz is the best all-around thermal jar I tested. Medium size, neutral appearance and top-notch thermal performance make this a long-lasting, solid choice for anyone.

The large size of the LunchBots 16 oz. jar makes it best for adults or older kids with large appetites. The wide mouth is the easiest to eat from.

Grown-ups and teens will appreciate the large capacity, classic stainless finish and excellent thermal performance.

The small size and easy-on and -off lid make this a easy to handle choice for young kids.

The small size and wide variety of designs — including favorite characters — makes the Funtainer a great choice for younger kids who are looking for cute lunch containers.

The shimmery finish of the pink and blue Zojurushi jars provide a pretty container for people looking for a bit of glamour and a high-quality lunch container. (The stainless jar is great for those who want a neutral appearance.)

* Disclosure: Though this post contains affiliate links, I have not been compensated in any way for writing this post or pulling this information together.

** My LunchBots jar shipped with a note saying that it should be hand-washed but I confirmed with the LunchBots team that the most recent version is dishwasher safe.

A few more notes:

Hot Water Test: For my baseline heat test, I began by filling each thermos with boiling water and letting it sit for 10 minutes to pre-heat. I then dumped out that water, filled the thermal jars with 204℉ water to the maximum fill line, put the lids on, and let the jars sit on my kitchen table for 4 hours. I chose 4 hours with the idea that a typical user might fill the thermos at 8 am and then eat lunch around noon. Of course, your situation may be different so adjust your expectations accordingly.

Hot Food Test: Filling a preheated thermal jar with near boiling water to the maximum fill line creates an ideal situation for heat retention but it doesn’t really reflect most real-world lunch-packing situations. For this test, I preheated the jars with boiling water for 10 minutes, then dumped it out and added 1 cup of baked beans and hot dog chunks (aka: beanie-weenies) to the thermos. This filled some jars to capacity (Thermos Foogoo and Funtainer) and only filled others about half-way (LunchBots 16 oz. and Stanley Adventurer) with the remaining jars somewhere in the middle. After 4 hours on my kitchen table the temperature of the food dropped between 44-59℉ depending on the jar used.

Cold Food Test: Don’t forget you can use a thermos to keep food cold too! For my cold food test, I began by placing each thermal jar in the freezer for 10 minutes with the lid off to pre-cool. Then I added 1 cup of yogurt (40℉, straight from the fridge), screwed the lid on and let the jars sit on my kitchen table for 4 hours. In that time, the temperature of the yogurt inside the jars rose between 9-15℉, with most coming in somewhere in the middle.

Did I miss something? Do you have a question about these thermoses that I didn’t answer? Please leave it in the comments below and I’ll do my best to find the answer.